Hafnium isotopes in zircon: A tracer of fluid-rock interaction during magnetite–apatite (“Kiruna-type”) mineralization

Abstract

In situ analyses of hafnium isotopes in zircon combined with U–Pb zircon ages, and trace element data, provide new information on the characterization and evolution of magnetite–apatite (“Kiruna-type”) deposits and the tectonic environments in which they occur. The Lyon Mountain granite in the Adirondack Mountains of New York is the host to numerous zircon-bearing magnetite–apatite deposits. Hafnium isotopic compositions and rare earth element contents in individual zircon crystals were measured in situ in both the host granites and the ore bodies by laser ablation inductively coupled plasma-mass spectrometry. Hafnium isotopic compositions in the ore zircon can be divided into two groups: those that have initial εHf (t) values that are indistinguishable from those of the host granites (e.g., εHf (t) less than +7) and are typical of relatively juvenile Proterozoic crust, and those that have extremely radiogenic εHf (t) values (as high +40). Two models are proposed to explain the observed εHf (t) values in the zircon crystals: 1) early-formed ore bodies containing magnetite, apatite, and clinopyroxene were remobilized by secondary fluid alteration, releasing Zr and Hf for the crystallization of new zircon; or alternatively, 2) fluids responsible for ore formation have interacted with garnet-bearing rocks during retrograde metamorphism, scavenging rare earth elements and radiogenic Hf.Previous work done to determine the U–Pb ages from the same zircon crystals, which were analyzed for the Hf isotopic composition in this study, revealed that ore bodies record a mineralizing event that is 20 to 60m.y. younger than the age of granite emplacement. This age discrepancy, plus the highly radiogenic εHf (t) values in the ore zircon crystals, suggests that the fluids responsible for this younger event could not have been derived from the granite hosts. These data argue that magnetite–apatite deposits in the LMG have multiple mineralizing events superimposed upon one another and that early-formed deposits may be reworked, modified and redeposited by fluids subsequent to magma crystallization.